47fi On the Creation, Diffusion, and 



from any central focus, but were placed " ab initio," in those 

 countries which were best adapted to them ; and also that 

 species have a real existence in nature, independent of the 

 fact, that however much a change of temperature might be 

 instrumental in changing or modifying the colours of a spe- 

 cies, it could never alter its actual form, — so that we know 

 that the Comma, and large tortoise-shell butterflies, although 

 of the same genus, could never have descended the one from 

 the other. 



But there is yet another class of beings to which the 

 remarks already applied to the insect world, may be still 

 better adapted, namely the terrestrial and fluviatile Mollusca. 



Species occur apparently in every country, however widely 

 they may be separated from each other, yet at the same 

 time many are extremely local, even in their respective coun- 

 tries, while others again have a wider range, and appear to 

 defy alike both heat and cold. 



Numerous species, for the most part peculiar to the Gan- 

 getic provinces of India, have of late years been brought to 

 notice by the researches of naturalists, while others again 

 have been discovered in the Sylhet and Himalayan mountain 

 ranges, with one or two exceptions quite distinct from the 

 lowland forms, and extending upwards to an elevation of 

 nearly fourteen thousand feet above the sea, upon the bor- 

 ders of eternal snows. 



It is interesting to observe the wide distribution of these 

 tender beings, and to trace the gradual, and almost imper- 

 ceptible yielding of one species to another, as the elevation, 

 or the climate varies. 



The shells of the lowland provinces give place in the 

 Himalaya, as the temperature becomes cooler, to forms 

 more nearly resembling those of Europe, some, still ad- 

 vancing a short way into the hills, but as if impatient of 

 the chills of a mountain winter, confining themselves to the 

 valleys of the lower ranges. 



